1. Field of the Invention
The present invention relates to a network system, and in particular to a network system composed of a master network synchronization device and a slave network synchronization device.
In recent years, a multimedia network system for the communication of a variety of data has been remarkable. Transmitting/receiving devices composing such a multimedia network system, like transmitting/receiving devices composing a conventional network, cannot receive information such as voice data from opposite devices if the devices are not operating at the same clock frequency as the opposite devices. Therefore, it is important to achieve a network synchronization which makes all of the devices composing the network system have a unified clock.
2. Description of the Related Art
FIG. 21 shows an arrangement of a conventional network system in which ATM switches 11-14 are connected in the form of a ring with routes 61-64. Among the ATM switches, the ATM switch 11 is a network synchronization master device, which performs a transmitting/receiving operation based on a clock 31 based on a clock 30 extracted from a signal from a route 60 connected to another network system (not shown), or based on a clock of a clock generator (not shown) of its own, i.e. the ATM switch 11 itself.
If no failure has occurred in any of the routes 61-64, the ATM switches 12-14 respectively extract the clocks 31_1, 32, and 31_2 from the routes 61, 62, and 64, which are first routes, to perform the transmitting/receiving operation in synchronization with the clock 31 of the ATM switch 11. Thus, a network synchronization is established for the entire network system.
Each of the ATM switches 12-14 has a second route set in lieu of the first route in case of a failure occurrence therein.
For example, if a failure occurs in the first route 61 (see timing T11 of FIG. 21), the ATM switch 12 which has detected this failure performs switchover of the clock extracting route (see timing T12 of FIG. 21) in order to extract the clock 33 from the route 62 which is the second route. Since the ATM switch 13 is extracting the clock from the ATM switch 12 at this time, a competition state occurs where the ATM switches 12 and 13 mutually extract their clocks from each other so that the synchronization is disturbed. As a result, the clocks of the ATM switches 12 and 13 run away or drive recklessly (see timings T13 and T14 of FIG. 21).
FIG. 22 shows a case where a failure occurs, not in the route 61 in the network system shown in FIG. 21, but in the ATM switch 11 itself which is the network synchronization master and causes the stoppage of the system (see timing T21 of FIG. 22).
The ATM switches 12 and 14 respectively detect the failure and perform the clock switchover from the clocks 31_1 and 31_2 of the first routes 61 and 64 to the clocks 33_1 and 33_2 of the second routes 62 and 63 (see timings T2 and T23 of FIG. 22).
Thus, the clocks of the ATM switches 12 and 13 run away (see timings T24 and T25 of FIG. 22) as is the case with a failure occurrence in the route 61 shown in FIG. 21 (see timings T13 and T14 of FIG. 21).
Moreover, when the clock of the ATM switch 13 runs away, the clock of the ATM switch 14 which has switched over to the clock of the ATM switch 13 also runs away (see timing T26 of FIG. 22), so that the network synchronization of the entire network system cannot be established.
For example, recent technologies of ATM (Asynchronous Transfer Mode) include a PNNI (Private Network Node Interface) in which two network interface functions called routing and signaling are defined.
Although this PNNI enables the data to be detoured in case a circuit (route) failure occurs, there is a possibility that the communication is interrupted since the clock is not detoured systematically.
Also, such a case where the data communication is interrupted due to the clock not being detoured systematically is not limited to ATM.
In the conventional network system, the above-mentioned problems have been dealt with the methods explained as follows:
(1)In case a failure occurs in the route 61 between the ATM switches 11 and 12 in FIG. 21, a maintenance operator of the ATM switch 12 switches over the route (port) for extracting the clock from the current second route to the third route 66 without problems to extract the clock 36.
At this time, the maintenance operator has to recognize the clock extraction status of not only the ATM switch 12 but also all of the adjoining ATM switches in order to select the port without problems. Therefore, the operation becomes extremely complicated.
(2)In case a failure occurs in the ATM switch 11 which is the network synchronization master in FIG. 22, and the master clock can not be extracted, the maintenance operator of the ATM switch 12 is required to review the clock of the entire network by making the clock of the ATM switch 12 the master clock after changing the clock extracting route to the third route 66, and to change the route from which the clock is extracted if there is an ATM switch requiring the change of the clock extracting route.
Thus, with the conventional methods, the maintenance operator has been greatly burdened when a failure occurs in the circuit or the own device.
It is an object of the present invention to provide a network system composed of a master network synchronization device and a slave network synchronization device wherein a network synchronization of the entire network system is established by selecting the clock extracting route without requiring a maintenance manual work when a failure occurs in the circuit or the master network synchronization device.
For the achievement of the above object, a master network synchronization device of a network system according to claim 1 of the present invention has clock routing means for repeatedly transmitting hop count information set to a predetermined initial value at a predetermined timing, and the slave network synchronization device has clock routing means for receiving the hop count information from an adjoining network synchronization device and for transmitting a minimum hop count between the slave and the master network synchronization device as the hop count information, a clock determination table for determining the minimum hop count between the slave and the master network synchronization device based on the received hop count information and for saving the minimum hop count and a route from which the minimum hop count is received as a clock extracting route, and a clock extractor for extracting a clock from the clock extracting route.
Namely, the network system is composed of the master network synchronization device and the slave network synchronization device, the network synchronization devices mutually transmitting the hop count information to adjoining network synchronization devices.
Therefore, this hop count information increases every time it is relayed by the network synchronization device.
The clock routing means of the master network synchronization device transmit the hop count information set to the initial value to the adjoining slave network synchronization device, for example, periodically.
The clock routing means of the slave network synchronization device provide the clock determination table with the received hop count information. The clock determination table sequentially determines the minimum hop count between its own device and the master network synchronization device from the received hop count information, and saves the minimum hop count and the route from which the minimum hop count is received as the clock extracting route.
The clock extractor extracts the clock from the signal on the clock extracting route saved to the clock determination table to establish the network synchronization.
Thus, the routes are systematically formed for sequentially transmitting the master clock from the slave network synchronization device nearer to the master network synchronization device (having smaller hop count) toward the slave network synchronization device farther from (having larger hop count) the master network synchronization device by taking the clock of the master network synchronization device as the master clock, so that no competition of the clocks occurs where the two network synchronization devices mutually extract their clocks from each other.
Also, in the network system according to claim 2, the slave network synchronization device in the above present invention may have a circuit failure detector for detecting a circuit failure, and the clock routing means of the slave network synchronization device may transmit failure occurrence information when a circuit failure on the clock extracting route is detected by the circuit failure detector or when the failure occurrence information indicating an occurrence of the circuit failure is received, and the clock determination table may save the failure occurrence information and, upon receiving the hop count information thereafter, update the hop count information and the route from which the hop count information is received as the minimum hop count and the clock extracting route, respectively.
Namely, the slave network synchronization device has a circuit failure detector for detecting a circuit failure. When the circuit failure detector detects the circuit failure in the clock extracting route, the clock routing means transmit the failure occurrence information indicating the occurrence of the circuit failure to the adjoining slave network synchronization device, and the clock determination table saves the failure occurrence information, thereby recognizing that its own device is not in the network synchronization state.
Moreover, the slave network synchronization device which has received the failure occurrence information transmits the failure occurrence information to the adjoining slave network synchronization device.
When the clock routing means of each of the slave network synchronization devices receive the hop count information thereafter, the clock determination table updates the received hop count information and the route from which the information is received as the minimum hop count and the clock extracting route, respectively, thereby recognizing that its own device is in the network synchronization state.
Thus, the clock extracting route excluding the route in which the circuit failure has occurred is systematically formed.
Namely, the clock determination table of each of the slave network synchronization devices updates the minimum hop count and the clock extracting route based on the received hop count information, so that the shortest detour of the clock extracting route excluding the route in which the failure has occurred is determined, thereby making the clock extractor extract the clock through the this detour clock route.
Moreover, if the route recovers from the failure, the hop count information through the recovered route is received by the adjoining network synchronization device. Since this route was originally the route with the minimum hop count, the clock extracting route is returned to the recovered route as in the case of claim 1.
Namely, it becomes possible to systematically switch over the clock extracting route upon the failure occurrence and the recovery therefrom.
Also, in the network system according to claim 3 of the present invention, the clock routing means in the above present invention of claim 1 may transmit the hop count information including metrics of the route.
Namely, the clock routing means make, quality and the like of the route (circuit) to which the hop count information is transmitted, the metrics, and transmit the hop count information including the metrics.
Thus, the clock extracting route formed systematically between the network synchronization devices can be made the route in which the quality and the like of the route transmitted therethrough are taken into account.
Also, in the network system according to claim 4 of the present invention, the clock determination table in the above present invention of claim 1 may determine the clock extracting route based on preset priorities when a plurality of routes having the minimum hop count exist.
Namely, when there are a plurality of routes having the minimum hop count, the clock determination table determines the clock extracting route therefrom based on the preset priorities. This enables the route with, for example, a higher reliability to be extracted as the clock extracting route.
Also, in the network system according to claim 5 of the present invention, the clock routing means of the slave network synchronization device in the above present invention of claim 2 may transmit the failure occurrence information as the hop count information, and the clock determination table may save the failure occurrence information as the minimum hop count and change the failure occurrence information to the hop count information received thereafter.
Thus, it becomes possible to include the failure occurrence information in the hop count information to make a single piece of hop count information, thereby simplifying the operation flow and deleting the information amount of transmission and reception.
Also, in the network system according to claim 6 of the present invention, the clock routing means in the above present invention of claim 1 may include an interface installing a routing protocol and a routing table, and the clock determination table may determine the minimum hop count and the clock extracting route based on metrics of the routing table.
Namely, the clock routing means of each of the network synchronization device may be composed of the interface installing a well-known routing protocol, such as the RIP (Routing Information Protocol) and the OSPF (Open Shortest Route First), and the routing table. In this case, the clock determination table determines the minimum hop count based on the hop count (relayed stage number) calculated from the metrics or the like included in the routing table, and makes the route from which the minimum hop count is received the clock extracting route.
Thus, it becomes possible to use the existing interface as the clock routing means in the network system composed of the device installing the interface of the well-known routing protocol, so that it becomes possible to simplify the arrangement of the device and lower the cost thereof.
Also, in the network system according to claim 7 of the present invention, when the master network synchronization device in the above present invention of claim 2 is made a first master network synchronization device, another network synchronization device may be made a second master network synchronization device, and when a system failure occurs in the first master network synchronization device, the second master network synchronization device may transmit the hop count information set to a predetermined initial value instead of the first master network synchronization device.
Namely, for the preparation of the case a failure occurs in the first master network synchronization device, the second master network synchronization device is preset as a substitute thereof.
When the system failure occurs in the first master network synchronization device, the clock routing means of the second master network synchronization device which has detected the system failure transmit the initialized hop count information to the adjoining slave network synchronization device.
Based on this hop count information, with the same operation as claim 1 of the present invention, the minimum hop count of the clock and the clock extracting route are set in the clock determination table of each slave network synchronization device.
Thus, the clock extracting route with the second master network synchronization device being made the master network synchronization device is systematically formed, so that a clock synchronized network is established.
It is to be noted that also in the network system which has the second master network synchronization device set, the circuit failure is detected and the detour of the clock is systematically established by the same operation as claim 2 of the present invention when the circuit failure occurs.
This can be applied to the case the second master network synchronization device is operating either as the network synchronization master or slave.
Also, in the network system according to claim 8 of the present invention, the clock routing means of the second master network synchronization device in the above present invention of claim 7 may detect the occurrence of the system failure by unreceiving subsequent hop count information before a predetermined time or more elapses after receiving the hop count information.
Namely, the first master network synchronization device, for example, periodically transmits the initialized hop count information. This hop count information reaches the second master network synchronization device directly or through the slave network synchronization device(s).
If the system failure occurs in the first master network synchronization device, the transmission of the hop count information is stopped. Accordingly, it becomes possible for the second master network synchronization device to detect unreceiving the subsequent hop count information before the predetermined time or more elapses after receiving the hop count information as the system failure.
Also, in the network system according to claim 9 of the present invention, each of the network synchronization devices in the above present invention of claim 7 may mount thereon an interface of a routing protocol which manages dynamic topology update information of a network, and the second master network synchronization device may recognize the system failure of the first master network synchronization device with the routing protocol.
Namely, each of the network synchronization devices mounts the interface of the routing protocol which manages the dynamic topology update information of the network system.
Normally, the interface of the first master network synchronization device, periodically transmits the topology update information with the routing protocol. The second master network synchronization device receives this topology update information directly or through another slave network synchronization device and recognizes that the system failure has not occurred in the first master network synchronization device.
If the system failure occurs, since the topology update information of the first master network synchronization device does not reach the destination, the second master network synchronization device recognizes the system failure of the first master network synchronization device and transmits the hop count information set to the predetermined initial value instead of the first master network synchronization device.
Also, in the network system according to claim 10 of the present invention, the slave network synchronization device in the above present invention of claim 9 may recognize the system failure of the first master network synchronization device with the routing protocol.
Namely, in the same way as the second master network synchronization device, the slave network synchronization device detects the system failure of the first master network synchronization device with the routing protocol, and recognizes itself as being not synchronizing with the network synchronization master clock.
Thus, the clock determination table can determine the minimum hop count based on the hop count information received thereafter to establish the network synchronization.
Also, in the network system according to claim 11 of the present invention, upon recognizing the system failure of the first master network synchronization device, the second master network synchronization device in the above present invention of claim 7 may establish the network synchronization with a clock extracted from a clock of a free-running clock generator in the device itself or a clock extracted from a predetermined route connected to a network system other than the network system to which the device itself belongs.
Thus, the second master network synchronization device is to use the clock independent of another network synchronization device in the network, so that the competition of the master clocks in the network is eliminated.
Also, in the network system according to claim 12 or 13 of the present invention, the clock extractor of each slave network synchronization device in the above present invention of claim 2 or 7 may perform only the communication for a specific route in synchronization with a clock from a specific route when the failure occurrence information is held in the clock determination table and communication errors occur on the specific route to which the hop count information is transmitted at a predetermined number of times or more, or at a predetermined time interval or shorter.
Namely, if the failure occurs in the circuit or the network synchronization device, the period, for which the synchronization of the free-running clock of each slave network synchronization device may be missed until the network synchronization is systematically re-established, corresponds to the period for which the failure occurrence information is held in the clock determination table. For this period, a possibility of communication error occurrence is high.
Accordingly, if the communication error occurs repeatedly in the specific route during the period for which the failure occurrence information is held in the clock determination table, the clock extractor performs only the communication between the opposite devices on the specific route until the network synchronization is re-established, in synchronization with the clock extracted from the specific route.
Thus, the communication errors which may occur until the network synchronization is re-established can be decreased.
Also, in the network system according to claim 14 of the present invention, the second master network synchronization device in the above present invention of claim 7 may operate as the slave network synchronization device, and the clock routing means stop transmitting the hop count information set to the initial value upon recognizing that the first master network synchronization device has recovered from the system failure.
Namely, the second master network synchronization device recognizes that the first master network synchronization device itself has recovered from the system failure by receiving, for example, the hop count information of the first master network synchronization device or the topology update information of the routing protocol.
Accordingly, the clock routing means of the second master network synchronization device stop transmitting the hop count information set to the initial value, and transmit the hop count information received from the adjoining network synchronization device including the minimum hop count instead. The clock extractor extracts the clock from the route with the minimum hop count and returns to the operation of the slave network synchronization device.
Thus, it is made possible to restore the master clock of the entire network synchronization from the clock of the second master network synchronization device to that of the first master network synchronization device when the first master network synchronization device has recovered from the system failure.
Also, in the network system according to claim 15 of the present invention, the clock routing means of the master network synchronization device in the above present invention of claim 7 may transmit a master device number and a master order of the device itself, the clock routing means of the slave network synchronization device may relay the master device number and the master order to an adjoining network synchronization device, and the clock determination table may determine the minimum hop count based on the hop count information, the master device number, and the master order. Namely, the first or the second master network synchronization device transmits the master device number and the master order of its own, when recognizing itself as being the master network synchronization device.
The clock routing means of the slave network synchronization device relay the received master device number and the master order to the adjoining network synchronization device. The clock determination table determines the minimum hop count based on the hop count information, the master device number, and the master order.
Thus, it is made possible for the slave network synchronization device to determine the minimum hop count more efficiently by discarding the unnecessary information and the like compared with the case of using the hop count information only.
Also, in the network system according to claim 16 of the present invention, the master network synchronization device in the above present invention of claim 1 may be connected to a route which is connected to another network system which is not included in the network system or to another device, and may extract a master clock from the route.
Thus, the master network synchronization device is made to use the clock independent of another network synchronization device included in the network system according to the present invention, thereby eliminating the competition of the master clocks in the network.
Also, it becomes possible to make the network system according to the present invention a sub-network system of another network system.
Also, the network system according to the present invention is enabled to include another network system. Moreover, it is made possible to connect a plurality of network systems according to the present invention.